1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for controlling transmission power of a reverse channel quality indicator channel.
2. Description of the Related Art
An IS-2000 CDMA (Code Division Multiple Access) mobile communication system, a typical mobile communication system, supports only a voice service. However, with the development of the communication technology and at the request of users, research has been carried out on a mobile communication system for supporting a data service. For example, an HDR (High Data Rate) communication system has been proposed to support only a high-speed data service.
The existing mobile communication systems are classified into one mobile communication system for supporting only the voice service and another mobile communication system for supporting only the data service. That is, although the mobile communication system is required to support both the voice service and the data service, the conventional mobile communication system is designed to support only one of the voice service and the data service. Therefore, there have been long demands for a mobile communication system capable of supporting both the voice service and the data service. To meet the demands, a 1×EV-DV (Evolution in Data and Voice) mobile communication system has been recently proposed.
A mobile communication system divides its whole service area into a plurality of cells, and manages the divided cells with their associated base stations (BSs). The base stations are centrally managed by a mobile switching center (MSC) so that a mobile station (MS) can continue a call while on the move between the cells. The base stations communicate with the mobile station over radio channels.
Unlike a land (or wired) communication system, the mobile communication system may have many transmission errors under the influence of fading and interference. A typical method for preventing the transmission errors is to increase transmission power high enough to cover the fading and interference. However, the increase in transmission power may interfere with radio channels of adjacent users. That is, in a mobile communication system, power control on a radio channel exerts a great influence on system performance. Commonly, a base station and a mobile station perform mutual power control. A procedure for controlling transmission power of a channel transmitted from the base station to the mobile station is called “forward power control,” while a procedure for controlling transmission power of a channel transmitted from the mobile station to the base station is called “reverse power control.”
In addition, a CDMA system can simultaneously connect a plurality of encoded channels through one frequency channel in the same time slot. By using this characteristic, a mobile station located in an overlapping region (or soft handoff region) between two adjacent base stations can simultaneously connect separate channels to the base stations in order to continue a call. This is called “soft handoff.” In this case, power control must be performed on all the base stations in communication with the mobile station.
FIG. 1 illustrates a method for performing reverse power control on a mobile station in a soft-handoff state in a conventional mobile communication system. Here, a mobile station 103 in a soft-handoff state communicates with two neighboring base stations (or sectors) 101 and 102.
A reverse power control procedure will be described with reference to FIG. 1. A sector #1 101 and a sector #2 102 each measure a reverse pilot channel (R-PICH) from the mobile 103 to estimate a signal-to-interference ratio Ep/Nt (i.e., a ratio of signal power of a pilot channel to interference power), and compare the Ep/Nt with a reference value (or outer loop set point) set for outer loop power control. If the estimated signal-to-interference ratio is larger than or equal to the reference value, the sectors 101 and 102 transmit power-down commands for a reverse channel to the mobile station 103 over a forward common power control channel (F-CPCCH). Otherwise, if the estimated signal-to-noise ratio is lower than the reference value, the base stations 101 and 102 send power-up commands for a reverse channel to the mobile station 103 over the forward common power control channel. The power control commands are transmitted by transmitting power control bits (PCBs) over a common power control channel (CPCCH) in the forward direction (i.e., F-CPCCH).
The mobile station 103 receives PCB1 and PCB2 over CPCCH1 and CPCCH2 from the sector #1 101 and the sector #2 102, respectively. If any one of the PCB1 and the PCB2 is a power-down command, the mobile station 103 decreases transmission power of a reverse channel, and if both of the PCB1 and the PCB2 are a power-up command, the mobile station 103 increases transmission power of the reverse channel.
Next, a forward power control procedure will be described. Transmission  power of a forward channel is determined using channel quality information received over a reverse channel quality indicator channel (CQICH). The channel quality indicator channel is used by the mobile station to measure a received signal strength indicator (e.g., carrier-to-interference ratio (C/I)) of forward common pilot channel transmitted from a specific sector, and transmit the C/I value to the sector.
In general, the mobile station measures C/I values of common pilot channels transmitted from neighboring sectors, and transmits quality information (e.g., the measured C/I value) of a sector (hereinafter, called a “best sector”) having the highest C/I value among the measured C/I values to the best sector (sector #1 101 in the case of FIG. 1) over the channel quality indicator channel. The best sector then transmits forward packet data to the mobile station.
Conventionally, transmission power of a channel quality indicator channel is set to maintain a specified ratio of a reverse pilot channel and a reverse traffic channel. That is, when transmission power of the reverse pilot channel and the reverse traffic channel is decreased, transmission power of the channel quality indicator channel is also decreased in a specific ratio. In contrast, if transmission power of the reverse pilot channel and the reverse traffic channel is increased, transmission power of the channel quality indicator channel is also increased in a specific ratio.
However, unlike the reverse pilot channel or traffic channel, the channel quality indicator channel is not a target of the soft handoff. That is, the channel quality indicator channel is received at only a specific sector having the best forward channel quality among the sectors neighboring to the mobile station. In contrast, since the reverse pilot channel and traffic channel are transmitted to two or more sectors in a soft handoff state, they can guarantee good reception performance. In addition, reception performance of the traffic channel can be improved through selection diversity or combining.
When power control on the channel quality indicator channel is performed in the same way as power control on the reverse pilot channel and traffic channel, reception performance of the reverse pilot channel and traffic channel satisfies a desired level, but there is a high possibility that reception performance of the channel quality indicator channel will be lower than the desired level. This will be explained further below with reference to FIG. 2.
In addition, in the case of the channel quality indicator channel, a sector measures reception power of the channel quality indicator channel, and performs an erasure process on the channel quality indicator channel if the measured reception power is not high enough to guarantee stable transmission. When the channel quality indicator channel is erased, a received signal on the channel quality indicator channel is not decoded. The sector then cannot acquire forward channel quality information, so the sector cannot normally perform forward power control. In addition, in a mobile communication system performing forward packet data transmission by consulting forward channel quality information like the 1×EV-DV system, frequent occurrence of the erasure process on the channel quality indicator channel causes a decrease in forward capacity. Therefore, there have been demands for power control on the reverse channel in order to improve performance of the channel quality indictor channel.